The market comes and goes. Sometimes, taxes make it impractical for a company to continue in business. Other times, companies have to raise their prices to accommodate taxes, so people do not want to buy the product anymore. Fiji will find other products, or they will simply live without the revenue.
Just like the tree that grows into the side of a mountain thousands of feet up, Fiji would survive without its number one export. Like all things it may be different, it may be harder and people would have to adjust. Nevertheless, humans have inhabited Fiji for generations and they will continue to do so, with or without their #1 export.
No, Fiji cannot survive without its number one export because people rarely buy it for anything but its water. If it is unable to remain competitive, Fiji will have to close up shop. This is the reality of business. They must adapt, otherwise they will have to move on to something else.
Fiji has other things to offer and will find a way to survive. People might not be able to buy Fiji water anymore, but Fiji is still a great place for tourism, and is a tropical island that many people like to visit and come enjoy the natural beauty of.